The pharmaceutical industry produces 25-100 kg or more of waste for every kg of active pharmaceutical ingredient (API) manufactured. According to a leading practitioner of the industry, the potential waste co-produced with APIs is in the range of 500 million to 2 billion kg/year. Thus, even at a nominal disposal cost of $1/kg, the potential savings associated with waste reduction would be significant compared to pharmaceutical industry's annual sales (US $500 billion in 2003). Most pharmaceuticals syntheses involve the use of homogeneous catalysts, which are difficult to be separated from the products. The resulting metal contamination of the products poses a serious concern in the pharmaceutical industry.
Heterogeneous catalysts may be more stable, cheaper, and easier to be separated from the products. However, their activity and selectivity are often lower than that of homogeneous catalysts. Thus, increasing efforts have been devoted towards developing efficient heterogeneous catalysts. Homogeneous catalysts that have been immobilised on a support would allow for the ease of catalyst recovery and reuse, and would minimize the waste generation and use of toxic chemicals, which is of great interest in the development of green chemical processes.
Palladium-catalyzed reactions have become an important tool in organic synthesis due to their high efficiency, selectivity, and diversity of possible transformations. Palladium-based catalysts have shown remarkable utility in coupling and hydrogenation reactions. However, despite their high activity, the homogeneous palladium-based catalysts suffer from low stability and high costs, which prevent their application in industrial processes. As a heavy metal, palladium is also highly undesirable as a contaminant of pharmaceutical products. To overcome these challenges associated with conventional palladium-based catalysts, heterogeneous and heterogenized catalysts have been developed. Palladium has been supported on materials such as carbon, zeolites, silicates and polymers for catalytic applications. Although these supported palladium catalysts allowed for ease of recovery, palladium leaching remained a significant problem. Additionally, to date polymer- or silica-supported Pd nanoparticles have shown only low chemical efficiency in catalytic applications.